1. Field of the Invention
The invention relates to techniques for testing and calibrating electromagnetic logging instruments adapted for subsurface measurements.
2. Background Art
Various well logging techniques are known in the field of hydrocarbon and water exploration and production. These techniques typically use instruments or tools equipped with sources adapted to emit energy into a subsurface formation that has been penetrated by a borehole. The emitted energy interacts with the surrounding formation to produce signals that are then detected and measured by one or more sensors on the instrument. By processing the detected signals, or profile of the formation properties is obtained.
Electromagnetic (EM) induction and propagation logging are well-known techniques. In this description an EM tool or EM logging tool is generally used to refer to either an induction or propagation-type tool because the present invention is applicable to both types. An EM logging tool is typically disposed within the borehole to measure the electrical conductivity (or its inverse, resistivity) of subsurface formations. A typical EM tool includes a transmitter antenna and one or more (typically a pair) receiver antennas disposed on the tool.
Antennas (or coils) may be operable as sources and/or sensors. For clarity, an EM logging tool in this description is described as having a certain transmitter-receiver configuration as if there were two distinct types of antennas—transmitter and receiver antennas. However, the principle of reciprocity applies, and one skilled in the art will appreciate that a transmitter and a receiver have similar physical/electrical characteristics and one can substitute for the other.
In both wireline and LWD applications, the antennas are mounted on the support member (mandrel or sonde) and axially spaced from each other along the longitudinal axis of the tool. These antennas are generally coils of the cylindrical solenoid type and are comprised of one or more turns of insulated conductor wire that is wound around the support member. In operations, the transmitter antennas is energized by an alternating current to emit EM energy through the borehole fluid (also referred to as “mud”) and into the formation. The signals detected at the receiver antenna(s) are usually expressed as a complex number (phasor voltage) and reflect interaction with the mud and the formation.
A coil carrying a current (e.g., a transmitter coil) generates a magnetic field that can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area of the coil. The direction and strength of the magnetic dipole can be represented as a vector perpendicular to the plane of the coil. The magnetic moment from the transmitter antenna is transmitted into the surrounding formation, which induces a current (eddy current) flowering in the formation around the transmitter. The eddy current in the formation in turn generates an magnetic field that induces an electrical voltage in the receiver antennas.
In conventional EM logging tools, the transmitter and receiver antennas are typically mounted with their axes aligned with the longitudinal axis of the instrument, i.e., these tools have longitudinal magnetic dipoles (LMD). When an LMD-equipped tool is placed in a borehole and the antenna is energized to transmit EM energy, eddy currents flow in loops in the surrounding formation in planes perpendicular to the borehole and there is no current flow up or down the borehole so long as the formation is axially symmetric about the tool axis.
An emerging technique in the field of well logging is the use of instruments incorporating tilted or transverse (at 90 degrees to the support axis) antennas, i.e., where the antenna's axis or magnetic moment is not parallel to the support axis. These instruments are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). One particular implementation uses a set of three coils having nonparallel axis (also referred to as “tri-axial”). The TMD antenna configuration can induce eddy currents in planes not perpendicular to the borehole and, thus, can provide EM measurements with direct sensitivity and sensitivity to the anisotropic resistivity properties of the formation. Logging instruments having TDMs are described in U.S. Pat. No. 6,163,155, 6,147,496, 5,115,198, 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, and 6,147,496.
Tool calibration is an important and necessary task of logging operations. Factors such as imperfections in tool construction and variations due to the tool's electronics (e.g., op-amp phase accumulations) will inevitably introduce errors in the measurements. Tool calibration provides a way to eliminate or compensate for the effects of these factors on the measurements data. Several methods are available for calibrating conventional LMD logging tools. For example, U.S. Pat. No. 4,876,511 discloses an external testing apparatus for testing and correcting an EM logging tool. This apparatus includes a shielded receiving devices that clamps around the transmitting antenna to intercept the transmitted signal and a shielded transmitting device that is positioned around the receiving antennas of the tool to transmit a signal which has a phase and/or amplitude related to the signal transmitted by the tool's transmitting antenna. This device simulates the effect that a geological formation would have on the signal if it were to travel from the tool's transmitting antenna through the formation. Because the simulated effect is known, the output of the tool may be verified or it may be corrected if necessary.
Other methods use conductive loops to calibrate the tool. When a transmitter is activated to emit EM energy, it induces an eddy current in the conductive loop. The eddy current in the conductive loop in turn induces an EM response to the receiver with an intensity that is a function of the eddy current magnitude. The measured signals are then used to calibrate the tool. Conventional conductive testing loops have magnetic dipoles oriented along the tool (z-oriented) axis to couple with LMD antennas only. U.S. Pat. No. 5,293,128 describes such as technique for calibrating an EM tool.
All of these methods are for the calibration of conventional LMD EM tools. With the advent of TMD-type logging tools with multiple antennas, however, calibration procedures require more complex measurements. TMD antenna configurations produce multiple couplings in multiple directions between transmitters and receivers. Thus a need exists for techniques to calibrate tools equipped with TMD antennas.